Pointers in C/C++

You may not be a C or C++ programmer and thinking there’s no need for this but knowledge of pointers is useful for any programmer irrespective of the language they use. Usually people tend to avoid pointers when they are learning c language because pointers are abstract thought of memory layout but it is a foundation of good programming principals. So, let’s understand what pointers are and how it works.

And before we get into knowing pointers, we have to know how memory works. See when we run a program we use memory to store variables and such. The memory a program uses is divided into blocks and these blocks are of different sizes of 32biots or 64bits depending on your operating system. All the blocks has a specific address associated with it and that’s why we can access every location of these blocks using its address. And as these addresses are in hexadecimal so, it is difficult to deal with them directly and that’s why we access memory in program through variables. Variables symbolically represents address in the memory and when we use variables we don’t worry about what the address is and that’s’ why we don’t have to worry about hexadecimal codes and we can focus on symbolic means.

In most of our programs we instantiate our variables but how’s that values get stored in memory? For example- we have two variables a and b which we’ll instantiate

int a = 5;

int b = 10;

Memory layout for these we’ll look something like this:

If we consider the entire memory broken down into blocks like above, we will find that the values of a and b we’ll occupy one of these blocks. And as the data type of a and b is integer(int), integers are 32bit that means 4bytes and if our OS is 32bits, we will find that integers occupy two of these blocks. So, we have 10 in one of the blocks and 5 in one of the box. Each of these blocks have addresses and addresses can be from zero to number of blocks our particular machine has or our program uses. One thing to remember here is the program decides which block we’ll occupy which variable values and we don’t have control over that. So, in short what is happening here is, ‘a’ and ‘b’ symbolically refers to a memory location, 5 and 10 are the values get stored in that locations and those lives in the memory spaces called a or b, these locations has underlying addresses that is known by our C program, we simply refers to those by ‘a’ or ‘b’. Now that we understand memory layout let’s move on to pointer.

What is a pointer?

We’ve just seen that when we initialized variable ‘a’, it’s assigned in some location of  the memory and the C program will decide  which location it can be assigned and symbolically we refer to the location as ‘a’.

But now we want to point to that location, what I mean by it is- for example we can set another variable aPtr which is located in completely different location in memory like below-

And aPtr holds the address of variable ‘a’

So, we can say ‘aPtr’ points to ‘a’ because , notice the value of ‘aPtr’ is 123FF which is the address of variable ‘a’. This is basically the core of how pointer works. A pointer just holds the address of another variable.

In short pointer is a special type of variable because regular variable stores integers, doubles etc. but pointers stores an address in memory. We can say a pointer points to another address in memory.

Now that we know what a pointer is, we have to know how to setup ‘aPtr’ to point to ‘a’.  

C implementation of pointer

it is just an one line command like this:

                                                         int *aPtr = &a;

Here int * means pointer to an integer that means int specific what kind of variable this pointer points to and * indicates that this is a pointer. The next thing here is the & symbol before the variable ‘a’,’ &’ indicates that we want the address of the variable not the value and we access the address of ‘a’ by that symbol(&) and assign it to ‘aPtr’.

Type of pointer variables

I have just given an example of  integer type pointer. But what other type do pointer variables have? We can point to any type of variable it can be primitive type(integer, double, character etc) as well as user defined types(structure in C). Type of a pointer depends on what it points to.

But remember earlier I have said addresses are just hexadecimal values, so now the question comes to mind that if all addresses are just hexadecimal values and pointer just holds the addresses, then why does it need to know what type of variable it points to? 

See all different variables occupies different amount of space in memory(i.e. int:4bytes,float: 4byes,char: 1byte) and pointer needs to know what type of variables it points to so that we can do something called pointer arithmetic to those variables. I will explain that on my next post.

Accessing the value the address the pointers holds

It's also just one line like this:

                                   int valueOfaPtr  =  *aPtr;

here we have a int type variable ‘valueOfaPtr’ which is initialized by the pointer variable ‘*aPtr’ that points the variable ‘a’ whose value is 5. By ‘*aPtr’ we are getting the value of the variable which address this (aPtr) variable is holding. So, now if we print the variable ‘valueOfaPtr’ we we’ll see that it has the value 5.

The whole code:

 The output:

First we have printed ‘a’ which has value 5 and after that we have printed ‘aPtr’ and it gave us a hexadecimal value that is the address of variable ‘a’ ad after that we have printed ‘*aPtr’ which give us the value 5 which is the value of ‘a’. Notice how ‘aPtr’ gives the address and ‘*aPtr’ gives the value, that’s why when we initialized the variable ‘valueOfaPtr’ we have initialized with ‘*aPtr’ because that is a normal variable we wanted to initialize that variable with value not the address.